The invention relates to a method and its use for coating sheets or sections by means of screen cylinders whose rotation is coordinated, where the sheets or sections are removed from a stack and sent by forced feed to the coating station. This invention also relates to a device for continuous precision coating of sheets or sections by means of screen cylinders and counter-pressure cylinders with a feed station and a horizontal feed device.
An enormous variety of coating sheets and sections are known today. On the one hand, there is the traditional offset and gravure printing, where photographic-quality results can be achieved. On the other hand, there is simple lacquering, with a large number of coating methods in between, for effect printings and special printings, e.g., with body inks, textured inks or fluorescent inks. Each consumer can select from packaging printers, gift paper manufacturers or the manufacturers of project folders and ring binders. Within the various coating and printing methods, screen printing or serigraphy, which has been known since early times, is still being used widely. Rotary web printing is often used today. The printing output with rotary web printing is much higher than that with web screen printing. The main disadvantage of rotary web printing is that only continuous paper can be processed from roll to roll at high throughput speeds. In addition, the processing width with respect to the paper is limited, usually being 20 to 60 cm. In gravure printing, both continuous webs and sheets or sections can be printed. The same thing is true of lacquering. The enormous variety of different processing techniques has the advantage that an optimum technique is available for each coating job. However, there is a great price increase with the individual techniques according to whether a small printing or a large printing is produced. This can lead to the situation where it costs only a fraction as much to produce a large printing of an advertising brochure as it does to print various reprints of portions of the large printing. For example, with a sales network of 30 sales personnel, it is often not economically feasible to generate a corresponding number of special address printings. Despite the very high level of technical advance throughout this industry, one encounters in many cases the paradoxical situation where mass printings are amazingly inexpensive, but each individually adapted small series is prohibitively expensive and even different processing companies must often be involved. The present inventors have recognized the fact that not only are screen printing, lacquering and coating with special materials such as adhesives similar techniques, but also there is a demand for a single processing department or company to be able to use these different techniques. Screen printing has the great advantage that relatively low forces are needed between the screen cylinder and the sheets. The main problem in rotary screen printing is that the movement between the screen cylinder and a printing cylinder opposite it must be synchronized with high precision during printing, so that the application to the sheets or sections can be made accurately. However, the beginning and end of the sheets to be printed must also exactly match the print original as obtained from the motion of the screen cylinder. The screen printing intended here is a finishing operation, often performed on a preprinted sheet. This may involve, for example, relief lacquering, where only narrowly defined areas within a sheet must be coated with a lacquer layer with a high precision. In the known state of the art, the sheets are held by grippers arranged with a space between them and secured to chains. The grippers are guided accurately during the printing operation. The counter-pressure rolls have recesses on the circumference for this purpose to allow the grippers to pass through. One of the main problems is satisfactory positioning of the objects relative to the printing roll and securing it during printing. Inaccurate placement or even deformation of the printed image must be avoided.
German Patent No. 693,644 is based on the object of developing a device that permits accurate positioning with regard to time and place along at least part of the path on which the objects are printed. It is proposed here that a short guide device be arranged with the highest precision within a continuous mechanical conveyor so that individual sheets can be moved in absolute synchronization with the printing cylinder. Unexamined German Patent No. 197 03 312 also concerns screen printing cylinder machines with a printing cylinder that is driven by a drive motor and receives the print material. The machine has a rotary screen cylinder which is axially arranged in parallel at a distance and is driven independently by a drive motor; it also concerns a device for synchronizing the motion of the printing cylinder and the screen cylinder during the printing operation. It has been found that the older printing method has some disadvantages with regard to acceleration of the printing cylinder, because this acceleration can cause the entire machine to vibrate. To solve this problem, it has been proposed that the printing cylinder, which has a circumference greater than or equal to the circumference of the screen cylinder, rotates at an essentially constant speed. The aforementioned patent cites as a great advantage the fact that the control expense for synchronization of the screen movement and the printing cylinder can be minimized due to the essentially constant rotational speed.
Both options have in common the fact that there are certain disadvantages, in particular with regard to mechanical controllability. Studies so far have shown that the state of the art has reached a very high quality standard with regard to flatbed printing. For example, distortion of sheets due to the effects of heat or moisture is often compensated in practice by corresponding distortion of the flatbed original or by replacing it with another flatbed original. This technique is difficult to apply to screen cylinders and would lead to complicated solutions. The present inventor has recognized that a concept must be developed to allow individual utilization of machines or a design for screen cylinders in particular, so that even smaller series can be coated inexpensively and economically in a high quality, and that one or more different operations of this type may optionally be carried out with one machine. The object of this invention was to improve upon the coating of sheets and sections so that they can be produced more rapidly and at a lower price. Another object was to increase the output from 7000 sheets or sections per minute as in the past to 10,000 per minute. According to the teaching of Dutch patent 9,201,676, the substrate such as paper or a textile which is to be printed is conveyed at a predetermined speed through the printing machine. The predetermined conveyance speed necessitates an adjustment of the rotational speed of the screen cylinder. This Dutch is based on material in the form of sheeting, such as paper or fabric which is normally wound onto rolls. However, the problems encountered with paper or fabric sheeting are completely different from those encountered with individual sheets and sections. The important task from the standpoint of the control technology is to make the rotational speed of the screen cylinder essentially the same as the conveyance speed.
The object of the present invention was to improve on the coating of sheets and sections such that the standard quality of flatbed printing can also be achieved with rotary screen printing with the highest possible output, so that sheets and sections can be coated inexpensively and economically in small printing runs in particular, and several and/or different techniques can be employed in one installation.
The method according to this invention is characterized in that the rotation of the screen cylinder can be adjusted and/or controlled with respect to the forced feed of sheets or sections, and in particular it can be corrected for a precisely coordinated run during the printing process and for synchronization between the sheets passing through and accurate coating and positioning of the coating on the sheets.
The device according to the present invention is characterized in that it has a precision work station with a constant conveyor for synchronization between the sheets passing through and accurate coating and positioning of the coating on the sheet, and the screen cylinder has a variable speed drive for referencing the screen cylinder with respect to print length and/or print position of the sheets or sections.
The present invention also relates to the use of this method or device and is characterized in that the machine is designed for screen printing as ink printing or lacquering sheets or sections or for applying special coatings such as adhesives or, with an appropriate design of the screen cylinder as a fast-change cylinder, e.g., as a screen printing cylinder, screen lacquering cylinder or as a special screen coating cylinder, where one or more screen cylinders can be used in one or more precision work stations.
The present invention has taken the previously rejected road in comparison with the known related art, namely the road of using an improved control technology. In many cases, an increased output or an improvement in quality can be achieved with great success by simply improving the purely mechanical aspects. However, there tend to be strict limits in this regard. A greater advance is made according to the present invention through better control of the motion of the screen cylinder. Surprisingly, several groups of problems can be solved in a new way by this discovery. The main goal is no longer absolute synchronization of the motion of the printing cylinder and the screen cylinder but instead it is to achieve the greatest possible synchronization between the sheets passing through and accurate coating or placement of the coating on the sheet. The present invention directly relates to the quality of the end product. As shown below, three problem areas can be approached successfully. It is known from experience that the length dimensions of sheets and sections can change due to the effects of moisture or heat. This can even occur while processing a stack. Frequently work is being done on a first stack while subsequent stacks are being stored outside the hot production room, e.g., at a higher or lower temperature or at a different atmospheric humidity. In the extreme case, the difference in length may amount to 0.5 to 1 millimeter. For accuracy in printing, however, a tolerance on the order of plus or minus one tenth of a millimeter is required. The sole requirement of synchronization of the revolution of the screen cylinder and the counter-pressure cylinder does not solve the problems described above. A frequent source of problems is working with several partial lots for the same job or having to divide the lots. Even if there are no differences in weight within a lot, such differences can occur with respect to the sheets or sections of different lots which are processed at longer intervals of time, especially due to the influencing factors mentioned above.
The present invention leads to a very new process technology. Transfer of the sheets from the stack to the forced feed for coating is handled as well as possible with mechanical means. As was also the case previously, the greatest possible accuracy is achieved through a gripper concept. The sheets are conveyed through the coating section by grippers. The disadvantage of the gripper concept is that it requires a considerable expenditure with regard to manufacturing costs. Although a conveyor belt concept would be much simpler, it would also be less accurate due to physical considerations. The accuracy in coating required above can now be achieved with the new concept through a setting or control and correction of the rotation of the screen cylinder. In a very few applications, grippers can be omitted, because accurate positioning of the sheets for coating is achieved by the screen cylinder with the control technology.
In the specific embodiment, control of rotation of the screen cylinder is based on a reference axis and is handled by a control unit which allows operation with or without correction of position. Position errors in individual forced-feed sheets or sections can be corrected by adjusting the starting printing position by controlling the rotation of the screen cylinder. For the duration of the printing operation, the peripheral speed of the screen drum is controlled with respect to the longitudinal repeat of the sheets, using acceleration and/or deceleration of the peripheral speed of the screen for referencing. The screen cylinder is preferably driven by a variable speed servo motor and controlled by a computer, where the position of the respective sheet or section fed to it relative to the motion of the screen cylinder is monitored by sensors at a distance before contact with the screen cylinder. It has also been found that the entire handling of the sheets or sections is optimum if the sheets or sections are conveyed toward and away from the coating station in a horizontal plane and if the sheets or sections are accelerated to the precise processing speed in their transfer from a feed point to the coordinated run during the printing operation.
The screen printing cylinder preferably has a variable speed servo drive, and the counter-pressure cylinder and the sheet transport through the printing machine have a variable speed drive, in particular a variable speed vector drive, which is paired directly with a local control unit or with one control unit for each drive. The entire system is controlled to a very great extent if the required settings or corrections according to the present invention are coordinated locally in the literal sense, i.e., among the elements involved. Therefore, it is proposed that in addition to machine control, a local control module with the individual control units is provided, where the setpoints are preselected by the machine control and the control corrections are made directly by the local control module.